Subject motion. For that shot, I wanted the stars and the clouds to remain stars and clouds rather than star trails and cloud trails. Of course I tried both though. This one is at ISO 400 and 520 s (pulled back nearly four stops in PP for the scene as a whole and a bit more than that in the sky but with the shadows slider at 100).

Another fantastic tip I hadn't thought of. I could have used that Sunday night. That is one thing I'm not too found of with that shot , the stars aren't spot on.

If you want to have the stars look like stars rather than something else, I'd say the max shutter speed is about 30 s. But that's on the border line and you might want to go down to about half that, as I did in the ISO 1600 shot you asked about.

A formula I've seen to get stars rather than star trails is:

Max Exposure Time = 600 / (focal length * sensor crop factor)

Some use 500 rather than 600.

I haven't tested.

I've already had that particular privilege. The "old" E-M5 was the first body to have that feature. See here for a few observations on how it works: